What is known as a mobile communication system is a system being composed of a base station control apparatus, a radio base station, and a mobile station like a CDMA (Code Division Multiple Access) radio network, in which variable rate transmission is performed over a downlink radio channel.
As illustrated in FIGS. 1 and 2, the present mobile communication system is composed of a base station control apparatus 4, a radio base station 5, and a mobile station 6.
In FIG. 2, the base station control apparatus 4 includes a transmission data buffering part 41 and a transmission data size determining part 42. The radio base station 5 includes a radio transmission rate determining part 51, a transmission frame producing part 52, and a transmitting part 53. The mobile station 6 includes a receiving part 61, a radio transmission rate determining part 62 and a received data decoding part 63.
FIG. 3 is a sequence chart showing an operation flow of variable rate transmission by the mobile communication system shown in FIG. 2.
Variable rate transmission is described with reference to FIGS. 2 and 3. In the base station control apparatus 4, downlink transmission data is temporarily stored in the transmission data buffering part 41 (b1 in FIG. 3). The transmission data size determining part 42 determines a transmission data size in accordance with the volume of the data stored in the transmission data buffering part 41 (b2 in FIG. 3), and transmits the transmission data of the determined data size to the radio base station 5 (b3 in FIG. 3).
In the radio base station 5, receiving the transmission data transmitted from the base station control apparatus 4, the radio transmission rate determining part 51 determines a radio transmission rate in accordance with the transmission data size and produces rate information (b4 in FIG. 3). The transmission frame producing part 52 encodes the transmission data into a transmission frame (b5 in FIG. 3). The transmitting part 53 transmits the encoded transmission frame and the determined rate information to the mobile station 6 (b6 in FIG. 3).
In the mobile station 6, the receiving part 61 receives a signal transmitted from the radio base station 5. The radio transmission rate determining part 61 either extracts the rate information included in the signal received at the receiving part 61 or estimates rate information from the power distribution of the received signal (b7 in FIG. 3). The received data decoding part 63 decodes the received data in accordance with the rate information estimated by the radio transmission rate determining part 62 (b8 in FIG. 3).
In the above-described mobile communication system, the radio base station 5 determines a radio transmission rate in accordance with a transmission data size and produces rate information to transmit the transmission frame and the rate information to the mobile station 6. The mobile station 6 decodes the received data in accordance with the extracted rate information or the estimated rate information.
It is known that in the mobile transmission system as described above, an error correction gain differs depending on a transmission data size. This is disclosed, for example, in TSG-RAN Working Group 1 meeting #5 TSGR1 #5 (99) 690, Cheju, South Korea, Jun. 1-4, 1999.
FIG. 4 shows the relation between each BER (Bit Error Rates) of 80, 160, 320, 640, 1280, 2560, 3840, and 5120 bits of the respective transmission data, and each Eb/No (Ratio of Energy per Bit to Noise Power Spectral Density) thereof. FIG. 5 shows the relation between each FER (Frame Error Rate) of 80, 160, 320, 640, 1280, 2560, 3840, and 5120 bits of respective transmission data, and each Eb/No thereof.
As evidenced by FIGS. 4 and 5, in the mobile communication system, if the transmission power from the radio base station 5 to the mobile station 6 is the same, the error rate of the small size of transmission data becomes higher than that of the large size of transmission data. This means that, if a transmission data size is small, a necessary error correction gain will not be obtained, causing degradation of reception quality of the mobile station.
It is a purpose of the present invention to provide a mobile communication system in which even if a transmission data size is small, a desired error correction gain can be obtained resulting in preventing the reception quality of the mobile station from deteriorating.
It is another purpose of the present invention to provide a radio base station for the mobile communication system.
It is yet another purpose of the present invention to provide a transmission power control method suited for the mobile communication system.